The present invention relates to a vacuum power interrupting device, and more particularly to a vacuum interrupting device adapted to be mounted on a power board or a switching board.
In our previous U.S. Pat. No. 4,365,127, issued Dec. 21, 1982, there is disclosed a vacuum power interrupter comprising a vacuum vessel consisting of a bell shaped metal casing, an insulating ceramic end plate hermetically brazed to the opening end of the metal casing, and a pair of contact rods disposed within the vacuum vessel so that one is in contact with the other or away therefrom.
This type of vacuum power interrupter makes it easy to enlarge an outer radius of the vacuum vessel and interrupt a large electric current, as compared with a second type of vacuum power interrupter comprising a cylindrical insulating envelope, metal end plates hermetically brazed to axial ends of the insulating envelope, respectively, and stationary and movable contact rods disposed within the insulating envelope so that the latter is movable relative to the former.
The first type of vacuum power interrupter is inexpensive and easy to fabricate since the insulating ceramic end plate is used instead of an insulating ceramic envelope which is costly.
However, with the first type of vacuum power interrupter, the following drawback occurs: an aerial creeping distance due to the electric potential which is rendered by the movable contact rod serving as an electrically charged portion through a bellows mounted on the end plate is obtained by an outer surface of the insulating end plate. Accordingly, the insulating withstanding force becomes small, resulting in that the insulating withstanding force is smaller than that of the second type of vacuum power interrupter. Consequently, the first type of vacuum power interrupter makes it difficult to interrupt high voltage.